In view of the recent trend towards preservation of the environment and space saving, the field of clinical photography has strongly desired a reduction in the quantity of processing waste. Thus, it has been required to develop the technology concerning photosensitive photothermographic materials which can achieve great effect in the exposure with a laser imagesetter or laser imager and produce clear black images having high resolution to enable the use as clinical photographs for assisting diagnosis and medical treatment. With such photosensitive photothermographic materials, the use of processing chemicals of solution type can be made needless and a simpler heat-development processing system causing no damage to the environment can be afforded to customers.
On the other hand, the semiconductor laser technology which has made expeditious progress in recent years enables the miniaturization of a medical image-output unit. As a matter of course, there have been developed the arts of producing infrared-sensitive (light- and heat-sensitive) silver halide photographic materials which make it possible to use a semiconductor laser as a light source. More specifically, the spectral sensitization arts applied to such photographic materials are disclosed in JP-B-03-10391 (The term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-06-52387, JP-A-05-341432 (The term "JP-A" at used herein means an "unexamined published Japanese patent application"), JP-A-06-194781 and JP-A-06-301141, and the anti-halation arts applied thereto are disclosed in JP-A-07-13295 and U.S. Pat. No. 5,380,635. In the photosensitive materials produced with a view to the exposure to infrared rays, the absorption of visible rays by sensitizing dyes and anti-halation dyes can be reduced to a great extent, so that it is easy to render such photosensitive materials colorless in a substantial sense.
However, dyes of the kind which can produce spectral sensitizing effect by the absorption of infrared rays generally have strong reducing power because of their high "HOMO", so that the fogging in photosensitive materials containing such dyes tends to be worsened due to reduction of the silver ions in the photosensitive materials. In particular, such dyes have a problem of causing a marked change in photographic properties upon storage under a high temperature-high humidity condition or for a long time. Conversely, the dyes of low "HOMO" employed for preventing the deterioration in keeping properties are inferior in spectral sensitizing efficiency because they are correlatively low in "LUMO", so that the photosensitive materials containing those dyes are low in sensitivity. Not only the photographic materials of wet process type encounter the problems concerning the sensitivity and the keeping properties as mentioned above, but also those problems become more serious in photothermographic materials relating to the present invention.
In addition, it is only natural that the reducing power produced by a dye has a tendency to be enhanced with an increase in the dye content. However, a decrease of the dye content results in a shortage of the sensitivity because the photosensitive material cannot sufficiently absorb the light incident thereon. In particular, photothermographic materials using highly lipophilic binders are deficient in sensitivity unless dyes are added thereto in sufficient amounts, because the adsorption of dyes to silver halide as a photosensitive element is weak therein.